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Prepares a connection over which a cancel request can be sent.

PGcancelConn *PQcancelCreate(PGconn *conn);

PQcancelCreate creates aPGcancelConn object, but it won't instantly start sending a cancel request over this connection. A cancel request can be sent over this connection in a blocking manner usingPQcancelBlocking and in a non-blocking manner usingPQcancelStart. The return value can be passed toPQcancelStatus to check if thePGcancelConn object was created successfully. ThePGcancelConn object is an opaque structure that is not meant to be accessed directly by the application. ThisPGcancelConn object can be used to cancel the query that's running on the original connection in a thread-safe way.

Many connection parameters of the original client will be reused when setting up the connection for the cancel request. Importantly, if the original connection requires encryption of the connection and/or verification of the target host (usingsslmode orgssencmode), then the connection for the cancel request is made with these same requirements. Any connection options that are only used during authentication or after authentication of the client are ignored though, because cancellation requests do not require authentication and the connection is closed right after the cancellation request is submitted.

Note that whenPQcancelCreate returns a non-null pointer, you must callPQcancelFinish when you are finished with it, in order to dispose of the structure and any associated memory blocks. This must be done even if the cancel request failed or was abandoned.

Requests that the server abandons processing of the current command in a blocking manner.

int PQcancelBlocking(PGcancelConn *cancelConn);

The request is made over the givenPGcancelConn, which needs to be created withPQcancelCreate. The return value ofPQcancelBlocking is 1 if the cancel request was successfully dispatched and 0 if not. If it was unsuccessful, the error message can be retrieved usingPQcancelErrorMessage.

Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.

Requests that the server abandons processing of the current command in a non-blocking manner.

int PQcancelStart(PGcancelConn *cancelConn);PostgresPollingStatusType PQcancelPoll(PGcancelConn *cancelConn);

The request is made over the givenPGcancelConn, which needs to be created withPQcancelCreate. The return value ofPQcancelStart is 1 if the cancellation request could be started and 0 if not. If it was unsuccessful, the error message can be retrieved usingPQcancelErrorMessage.

IfPQcancelStart succeeds, the next stage is to polllibpq so that it can proceed with the cancel connection sequence. UsePQcancelSocket to obtain the descriptor of the socket underlying the database connection. (Caution: do not assume that the socket remains the same acrossPQcancelPoll calls.) Loop thus: IfPQcancelPoll(cancelConn) last returnedPGRES_POLLING_READING, wait until the socket is ready to read (as indicated byselect(),poll(), or similar system function). Then callPQcancelPoll(cancelConn) again. Conversely, ifPQcancelPoll(cancelConn) last returnedPGRES_POLLING_WRITING, wait until the socket is ready to write, then callPQcancelPoll(cancelConn) again. On the first iteration, i.e., if you have yet to callPQcancelPoll(cancelConn), behave as if it last returnedPGRES_POLLING_WRITING. Continue this loop untilPQcancelPoll(cancelConn) returnsPGRES_POLLING_FAILED, indicating the connection procedure has failed, orPGRES_POLLING_OK, indicating cancel request was successfully dispatched.

Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.

At any time during connection, the status of the connection can be checked by callingPQcancelStatus. If this call returnsCONNECTION_BAD, then the cancel procedure has failed; if the call returnsCONNECTION_OK, then cancel request was successfully dispatched. Both of these states are equally detectable from the return value ofPQcancelPoll, described above. Other states might also occur during (and only during) an asynchronous connection procedure. These indicate the current stage of the connection procedure and might be useful to provide feedback to the user for example. These statuses are:

Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these occurring in a particular order, or at all, or on the status always being one of these documented values. An application might do something like this:

switch(PQcancelStatus(conn)){        case CONNECTION_STARTED:            feedback = "Connecting...";            break;        case CONNECTION_MADE:            feedback = "Connected to server...";            break;...        default:            feedback = "Connecting...";}

Theconnect_timeout connection parameter is ignored when usingPQcancelPoll; it is the application's responsibility to decide whether an excessive amount of time has elapsed. Otherwise,PQcancelStart followed by aPQcancelPoll loop is equivalent toPQcancelBlocking.

Returns the status of the cancel connection.

ConnStatusType PQcancelStatus(const PGcancelConn *cancelConn);

The status can be one of a number of values. However, only three of these are seen outside of an asynchronous cancel procedure:CONNECTION_ALLOCATED,CONNECTION_OK andCONNECTION_BAD. The initial state of aPGcancelConn that's successfully created usingPQcancelCreate isCONNECTION_ALLOCATED. A cancel request that was successfully dispatched has the statusCONNECTION_OK. A failed cancel attempt is signaled by statusCONNECTION_BAD. An OK status will remain so untilPQcancelFinish orPQcancelReset is called.

See the entry forPQcancelStart with regards to other status codes that might be returned.

Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.

Obtains the file descriptor number of the cancel connection socket to the server.

int PQcancelSocket(const PGcancelConn *cancelConn);

A valid descriptor will be greater than or equal to 0; a result of -1 indicates that no server connection is currently open. This might change as a result of calling any of the functions in this section on thePGcancelConn (except forPQcancelErrorMessage andPQcancelSocket itself).

Returns the error message most recently generated by an operation on the cancel connection.

char *PQcancelErrorMessage(const PGcancelConn *cancelconn);

Nearly alllibpq functions that take aPGcancelConn will set a message forPQcancelErrorMessage if they fail. Note that bylibpq convention, a nonemptyPQcancelErrorMessage result can consist of multiple lines, and will include a trailing newline. The caller should not free the result directly. It will be freed when the associatedPGcancelConn handle is passed toPQcancelFinish. The result string should not be expected to remain the same across operations on thePGcancelConn structure.

Closes the cancel connection (if it did not finish sending the cancel request yet). Also frees memory used by thePGcancelConn object.

void PQcancelFinish(PGcancelConn *cancelConn);

Note that even if the cancel attempt fails (as indicated byPQcancelStatus), the application should callPQcancelFinish to free the memory used by thePGcancelConn object. ThePGcancelConn pointer must not be used again afterPQcancelFinish has been called.

Resets thePGcancelConn so it can be reused for a new cancel connection.

void PQcancelReset(PGcancelConn *cancelConn);

If thePGcancelConn is currently used to send a cancel request, then this connection is closed. It will then prepare thePGcancelConn object such that it can be used to send a new cancel request.

This can be used to create onePGcancelConn for aPGconn and reuse it multiple times throughout the lifetime of the originalPGconn.

Creates a data structure containing the information needed to cancel a command usingPQcancel.

PGcancel *PQgetCancel(PGconn *conn);

PQgetCancel creates aPGcancel object given aPGconn connection object. It will returnNULL if the givenconn isNULL or an invalid connection. ThePGcancel object is an opaque structure that is not meant to be accessed directly by the application; it can only be passed toPQcancel orPQfreeCancel.

Frees a data structure created byPQgetCancel.

void PQfreeCancel(PGcancel *cancel);

PQfreeCancel frees a data object previously created byPQgetCancel.

PQcancel is a deprecated and insecure variant ofPQcancelBlocking, but one that can be used safely from within a signal handler.

int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);

PQcancel only exists because of backwards compatibility reasons.PQcancelBlocking should be used instead. The only benefit thatPQcancel has is that it can be safely invoked from a signal handler, if theerrbuf is a local variable in the signal handler. However, this is generally not considered a big enough benefit to be worth the security issues that this function has.

ThePGcancel object is read-only as far asPQcancel is concerned, so it can also be invoked from a thread that is separate from the one manipulating thePGconn object.

The return value ofPQcancel is 1 if the cancel request was successfully dispatched and 0 if not. If not,errbuf is filled with an explanatory error message.errbuf must be a char array of sizeerrbufsize (the recommended size is 256 bytes).

PQrequestCancel is a deprecated and insecure variant ofPQcancelBlocking.

int PQrequestCancel(PGconn *conn);

PQrequestCancel only exists because of backwards compatibility reasons.PQcancelBlocking should be used instead. There is no benefit to usingPQrequestCancel overPQcancelBlocking.

Requests that the server abandon processing of the current command. It operates directly on thePGconn object, and in case of failure stores the error message in thePGconn object (whence it can be retrieved byPQerrorMessage). Although the functionality is the same, this approach is not safe within multiple-thread programs or signal handlers, since it is possible that overwriting thePGconn's error message will mess up the operation currently in progress on the connection.